Change of career?

My son is half way through his training in San Antonio to be a hospital corpsman.  He has decided that this is definitely not for him and wants to be a boatswain's mate.  When he enlisted, his recruiters told him that the only available jobs were hospital corpsman, which he later learned was not true.  He has also been told that he will be shipped out with Marines and will have to go through Marine basic Training.  He has recently  been having trouble with his knees and knows he won't make it through Marine BT.  What are his chances of switching jobs/careers at this stage?